the present invention relates to a method of operating a fuel cell system and, more particularly, to treatment of a combustible gas filled in a cathode of a fuel cell in the fuel cell system after stop of power generation.
Referring to FIG. 9, a fuel cell system 39 comprises, as main components, a polymer electrolyte fuel cell (PEFC) 1 provided with an anode 1a and a cathode 1cal , a fuel generator 2 configured to reform a feed gas for power generation such as a city gas or a natural gas by addition of water to generate a hydrogen-rich fuel gas, a water supply device 3 configured to supply water to the fuel generator 2, a feed gas supply device 36 configured to supply the feed gas to the fuel generator 2, a combustor 4 configured to combust a fuel gas unconsumed in the anode 1a of the fuel cell 1 and exhausted therefrom, a blower 5 which is an oxidizing gas supply device configured to supply an oxidizing gas (air) to the fuel cell 1 and to discharge the oxidizing gas outside the fuel cell 1, and a purge air supply device 6 configured to supply purge air to the fuel generator 2 to perform purging from the interior from the fuel generator 2 when the fuel cell system 39 stops power generation.
The fuel cell system 39 is configured to generate electric power by reacting the hydrogen-rich fuel gas supplied to the anode 1a and the air supplied to the cathode 1c as the oxidizing gas within the fuel cell 1. In a stop operation of the fuel cell system 39, finally, purging of fuel gas passages is conducted by using the purge air. The fuel cell system 39 is configured to carry out the power generation operation and the stop operation under the condition in which a controller 21 properly controls the blower 5, the feed gas supply device 36, the water supply device 3, the air supply device 6, etc.
In the fuel cell system 39, a cost can be reduced because of the absence of a nitrogen storage device, as compared to the conventional purging process in which, when the fuel cell system stops the power generation, nitrogen, instead of the feed gas, is flowed from a feed gas supply passage to the fuel generator and to the fuel cell to purge the gases (e.g., fuel gas) remaining within these components to the combustor, which treats the purged gases.
A fuel cell system which employs such air purging technique is disclosed in Womb 01/97312. In this fuel cell system, when the fuel cell stops power generation, water is supplied from a water supply device to a fuel generator to generate steam, which is used to purge a fuel gas containing hydrogen remaining within fuel gas passages, and thereafter, the air is flowed from a purge air supply device to finally perform purging from the interior of the fuel gas passages. In this fuel cell system, hydrogen of the fuel cell is driven out by the steam and then the air is supplied to the interior of the fuel cell. This makes it possible to inhibit corrosion of the passages which may be caused by water droplets resulting from steam condensation.
An anode of a polymer electrolyte fuel cell is typically comprised of alloy catalyst containing platinum and ruthenium. If the anode is exposed to air as in the conventional fuel cell system, catalyst may be oxidized by oxygen and possibly degrade catalytic performance (i.e., oxidization and degradation of the anode may take place). Considering maintaining durability of the fuel cell system, it is undesirable to fill and keep the air in the anode when the fuel cell system stops power generation.
In order to solve this problem, when the fuel cell system stops power generation, the fuel gas (e.g., hydrogen-rich fuel gas) or the feed gas (city gas or natural gas) is filled and confined in the anode of the fuel cell to inhibit entry of the air into the anode, thereby maintaining durability of the fuel cell.
In this method, apparently, oxidization and degradation of the anode may be inhibited. But, if the air (oxidizing gas) remains in the cathode after the stop of the power generation in the fuel cell system, it flows to the anode through a porous polymer electrolyte membrane and thereby the anode may be oxidized and degraded. Inventors or the like considers that the cathode is also required to be filled with the fuel gas or the feed gas to reliably avoid oxidization and degradation of the anode.
However, in a case where the cathode of the fuel cell is filled with the fuel gas or the feed gas after the stop of the power generation, the combustible feed gas or fuel gas is discharged to atmosphere when the air is supplied from an air supply system to the cathode of the fuel cell at the start of a next operation of the fuel cell.